Question: Which of these numbers is prime? ${15,\ 21,\ 45,\ 50,\ 59}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 21 are 1, 3, 7, and 21. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 50 are 1, 2, 5, 10, 25, and 50. The factors of 59 are 1 and 59. Thus, 59 is a prime number.